Medical apparatus incorporating pressurized supply of storage liquid

ABSTRACT

An improved apparatus is disclosed for maintaining a medical assembly, e.g., a blood chemistry electrode assembly, filled with a bubble-free storage liquid for an extended storage time period. The apparatus achieves this result by connecting to the assembly a pressurized reservoir that supplies additional storage liquid, to replenish any storage liquid lost due to diffusion from the medical assembly.

BACKGROUND OF THE INVENTION

This invention relates generally to medical apparatus for use inextracting liquids from, or injecting liquids into, a living body and,more particularly, to medical apparatus of this kind that are filledwith a storage liquid for extended time periods.

Medical apparatus of this particular kind can take many forms, includingthat of a blood chemistry sensor assembly of the kind disclosed in U.S.Pat. No. 5,165,406, issued in the name of David K. Wong and entitled"Electrochemical Sensor Apparatus and Method." The disclosed bloodchemistry sensor assembly includes a rigid plastic housing that definesan elongate test chamber through which passes either blood or acalibration fluid. A plurality of electrochemical sensors are locatedwithin the housing, adjacent to the test chamber, each sensor beingadapted to measure a separate parameter of the adjacent fluid, e.g.,oxygen partial pressure (pO₂), carbon dioxide partial pressure (pCO₂),glucose, hematocrit, calcium, hydrogen ion (i.e., pH), chloride,potassium, and sodium.

Prior to its being used to measure various parameters of a blood sample,the sensor assembly described briefly above typically is stored for anextended time period, with a storage liquid being located within theassembly's test chamber and with the entire assembly being stored withina sealed container of low permeability. The presence of the storageliquid prevents the sensors from drying out, which could adverselyaffect their sensing capability, even if subsequently re-wetted. Overtime, however, the liquid can diffuse from the test chamber, to at leasta limited extent, through the assembly's plastic housing and associatedtubing, etc.

The diffusion of liquid from the test chamber can give rise to twoproblems. First, the resulting negative pressure can draw air or othergas into the test chamber, to form bubbles in the remaining liquid.These gas bubbles can contribute to drying of the sensors, whichadversely affects the sensors' performance, even if the bubbles areflushed out prior to use. Further, the diffusion of liquid from the testchamber can change the composition of the remaining liquid. In manycircumstances, this change can adversely affect the accuracy of thesubsequent sensor measurements.

Other kinds of medical apparatus incorporating liquid-filled chambersthat must be stored for extended time periods include ready-mixmedication containers configured to directly administer the medicationto a patient intravenously, and reagent-containing diagnostic kits forvarious analytes, etc.

It should, therefore, be appreciated that there is a need for medicalapparatus of the kind described above, wherein the apparatus can bereliably stored over an extended time period, without risk of air orother gas entering the apparatus' chamber., and without substantialchange to the composition of the storage liquid. The present inventionfulfills this need and provides further related advantages.

SUMMARY OF THE INVENTION

The present invention is embodied in an improved medical apparatus foruse in extracting a liquid from, and/or injecting a liquid into, aliving body, wherein the apparatus is configured to ensure that air andother gases do not enter a liquid-filled chamber while the apparatus isstored over an extended time period. More particularly, the medicalapparatus includes a housing that defines the liquid-filled chamber,with various components of the apparatus having at least limitedpermeability, to allow liquid to diffuse from the housing chamber overtime. To compensate for this limited diffusion, the apparatus includes aliquid reservoir in communication with the housing chamber and carryinga storage liquid, and further includes a spring-bias device forpressurizing the liquid reservoir, to pressurize the housing chamberwith the storage liquid, such that additional storage liquid isavailable to replenish liquid that might diffuse from the chamber overtime.

In other, more detailed features of the invention, the medical apparatusfurther includes a one-way check valve located between the liquidreservoir and the housing chamber, for limiting the flow of liquid fromthe chamber to the reservoir. In addition, the chamber and the reservoirare part of a sterile system, closed to the external environment, andthe apparatus further includes an outer container for carrying thehousing, the reservoir, and the spring-bias device. Further, thereservoir is sized relative to the housing chamber such that thecomposition of the storage liquid remains substantially unchanged overtime.

In one embodiment of the invention, the medical apparatus' spring-biasdevice includes a substantially rigid body that defines a cavity sizedto carry the liquid reservoir, which takes the form of a flexible bag,and further includes a spring-biased plunger for compressively engagingthe flexible bag. In an alternative embodiment, the spring-bias deviceincludes a spring clip sized to compressively grasp opposite sides ofsuch a flexible bag.

The medical apparatus of the invention has particular utility when ittakes the form of a sensor apparatus incorporating one or moreelectrochemical sensors in operative contact with the liquid stored inthe housing chamber. Use in the form of a blood chemistry sensorassembly is particularly advantageous.

Other features and advantages of the present invention should becomeapparent from the following description of the preferred embodiments,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a combination infusion fluid deliveryand blood chemistry analysis system that incorporates an electrochemicalsensor assembly that is part of the preferred embodiment of theinvention, shown connected to a blood vessel of a patient's arm.

FIG. 2 is a cross-sectional view of the electrochemical sensor assemblyof FIG. 1, showing several electrochemical sensors located adjacent tothe assembly's test chamber.

FIG. 3 is a schematic view of the electrochemical sensor assembly ofFIG. 1, combined with a pressurized reservoir assembly in accordancewith one preferred embodiment of the invention.

FIG. 4 is a sectional isometric view of the pressurized reservoirassembly of FIG. 2, showing a spring-biased platen that compressivelyengages a flexible bag carrying a supply of storage liquid.

FIG. 5 is a plan view of the flexible bag of FIG. 4, shown with anattached delivery tube and one-way check valve.

FIG. 6 is a perspective view of a spring clip that can be used tocompressibly engage a flexible bag to form a pressurized reservoirassembly in accordance with an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the drawings, and particularly to FIGS. 1 and 2,there is shown an infusion fluid delivery and blood chemistry analysissystem in use connected to a blood vessel within a patient's arm 11. Aninfusion pump 13, under the control of a controller 15, pumps aninfusion/calibration fluid from a fluid source 17 to the blood vesselvia an infusion tube 19 and hollow needle 21. A sensor/electrodeassembly 23 is located in the infusion line, the assembly including atest chamber 25 through which the infusion/calibration fluid passes onits way to the patient. Several electrochemical sensors 27 are locatedwithin the electrode assembly, immediately adjacent to the test chamber,such that they can measure predetermined parameters of the adjacentfluid.

Periodically, the controller 15 conditions the pump 13 to interrupt itspumping of the infusion/calibration fluid to the patient and, instead,to reverse direction and draw a blood sample from the patient. Thisblood sample is drawn rearwardly through the infusion tube 19 as far asthe electrode assembly 23, to allow the assembly's electrochemicalsensors 27 to measure certain parameters of the blood. After themeasurements have been completed, the pump reinfuses the blood sampleback into the patient and then resumes pumping the infusion/calibrationfluid. The sensors are configured to measure parameters such as oxygenpartial pressure (pO₂), carbon dioxide partial pressure (pCO₂), glucose,hematocrit, calcium, hydrogen ion (i.e., pH), chloride, potassium, andsodium.

The electrode assembly 23 normally must be stored for an extended timeperiod before being used to make blood sample measurements. During thisstorage time, it is important that the electrochemical sensors 27 remainwetted, because dried-out sensors can provide inaccurate measurements,even if re-wetted prior to making their measurements. The electrodeassembly, therefore, is stored with a storage liquid filling its testchamber 25. Unfortunately, the plastic materials from which the body ofthe electrode assembly and the infusion tube are constructed allow atleast limited diffusion of the storage liquid from the test chamber.Unless appropriately controlled, this diffusion would lead to a negativepressure within the test chamber, thus drawing air or other gases intothe remaining liquid and causing undesired bubbles to be formed.

As shown in FIG. 3, this air bubble problem is avoided by storing theelectrode assembly 23 with a pressurized reservoir assembly 29, whichprovides a pressurized supply of storage liquid to replenish whateverliquid is lost through diffusion. The electrode assembly and thepressurized reservoir assembly form a closed, sterile system thatcarries the storage liquid and isolates the liquid from the externalenvironment. The reservoir assembly incorporates a spring-loaded checkvalve 31, for coupling directly to a standard luer connector in theinfusion line 19 that projects from the electrode assembly. The valveopens only when connected to the mating luer connector, thus allowingthe reservoir assembly to be disconnected from the electrode assemblywithout a loss of the storage liquid. The two assemblies are packagedwithin a sealed container 33, preferably of low permeability. Thereservoir assembly is sized to carry sufficient storage liquid toreplenish the amount of liquid expected to be lost due to diffusion overthe maximum storage time period.

FIGS. 4 and 5 depict the pressurized reservoir assembly 29 in greaterdetail. The assembly includes a flexible bag 35 formed from two flatplastic sheets arranged in confronting relationship and heat-sealedaround their periphery. The bag is carried in a cavity 37 of a hollowhousing 39, and a removable cover 41 encloses the cavity. A moldedplastic, cup-shaped platen 43 is positioned between the bag and thecover, and a coil spring 45 urges the platen into compressive engagementwith the bag. Cross braces 47 are molded into the platen's top side, toprovide rigidity and to define a ring-shaped recess 49 for receiving thecoil spring.

A plastic delivery tube 51 extends from one edge of the flexible bag 35through an opening 53 in the removable cover 41 to the check valve 31.The flexible bag preferably is formed of polypropylene copolymer, withthe strength of the peripheral seal being at least 5 pounds per inch,and with the bag able to withstand a pressure of 6 pounds per squareinch for at least 48 hours without leakage. The delivery tube preferablyincludes two coextruded layers, with an outer layer formed of polyolefinand an inner layer formed of polyvinyl chloride (PVC). The check valvepreferably is a conventional Halkey-Roberts luer-activated valve, formedof PVC, and it is bonded to the tube using cyclohexanone, with a bondstrength of at least 10 pounds.

FIG. 6 depicts a flat metal spring clip 55 that can be used with aflexible bag 35 like that of FIG. 5, to form an alternative embodimentof a pressurized reservoir assembly. The spring clip is sized andconfigured to slide over the bag and pressurize the storage liquidcontained within it. In this embodiment, the bag is formed of a materialhaving low water vapor permeability, to reduce fluid loss.

In other embodiments, not shown in the drawings, the pressurizedreservoir assembly can incorporate alternative means for maintainingfluid pressure. Examples of such alternative pressure means includecompressed air or other gas, compressible foam, and other extension andcompression spring arrangements.

In the preferred embodiments, the flexible bag 35 is substantially lesspermeable than is the sensor/electrode assembly 23. Because of this, thestorage liquid diffuses away from the sensor/electrode assembly fasterthan it does from the bag. In addition, the bag preferably is sized tocarry a substantially greater amount of storage liquid than is stored inthe test chamber 25. Consequently, as the storage liquid within the testchamber is replenished with liquid from the bag, its composition remainssubstantially unchanged. This is a significant improvement over priorsystems of this kind, in which the diffusion of liquid from the testchamber sometimes had a significant (and undesired) effect on thecomposition of the liquid that remains.

It should be appreciated from the foregoing description that the presentinvention provides an improved apparatus for maintaining a medicalapparatus, e.g., an electrochemical sensor assembly, filled with astorage liquid and bubble-free for an extended storage time period. Theapparatus achieves this result by connecting to the sensor assembly apressurized reservoir that supplies additional storage liquid toreplenish any storage liquid lost due to diffusion from the medicalassembly.

Although the invention has been described in detail with reference onlyto the presently preferred embodiments, those of ordinary skill in theart will appreciate that various modifications can be made to thoseembodiments without departing from the invention. Accordingly, theinvention is defined only by the following claims.

We claim:
 1. Medical apparatus adapted to be stored for an extended timeperiod, the apparatus comprising:a housing defining a liquid-filledchamber, wherein the housing is configured such that liquid locatedwithin the chamber can escape therefrom over time; a sensor carriedwithin the housing, in operative relationship to the chamber, formeasuring the concentration of a predetermined parameter of any liquidwithin the chamber; a liquid reservoir communicating with the housingchamber and carrying a storage liquid; and a spring-bias device forpressurizing the liquid reservoir, to pressurize the chamber with thestorage liquid, such that additional storage liquid is supplied to thechamber as liquid escapes therefrom over time.
 2. Medical apparatus asdefined in claim 1, and further comprising a one-way check valve locatedbetween the liquid reservoir and the housing chamber, for limiting theflow of liquid from the chamber to the reservoir.
 3. Medical apparatusas defined in claim 1, wherein:the housing chamber and the liquidreservoir define a closed, sterile system; and the medical apparatusfurther comprises an outer container for carrying the housing, thereservoir, and the spring-bias device, the outer container beingsubstantially liquid impermeable.
 4. Medical apparatus as defined inclaim 1, wherein:the liquid reservoir is a liquid-filled flexible bag;and the spring-bias device includesa substantially rigid body defining acavity sized to carry the liquid-filled flexible bag, and aspring-biased plunger for compressively engaging the liquid-filledflexible bag.
 5. Medical apparatus as defined in claim 1, wherein:theliquid reservoir is a liquid-filled flexible bag; and the spring-biasdevice includes a spring clip sized to compressively grasp oppositesides of the liquid-filled flexible bag.
 6. Medical apparatus as definedin claim 1, wherein the components of the medical apparatus aresufficiently permeable to allow limited escape of the storage liquidtherefrom over time and to allow air to be introduced into the chamberover time.
 7. Sensor apparatus for sensing the concentration of apredetermined parameter in a liquid, comprising:a housing defining atest chamber; a sensor carried within the housing, in operativerelationship to the test chamber, for measuring the concentration of apredetermined parameter of any liquid located within the test chamber;wherein the apparatus is configured such that liquid located within thetest chamber can escape therefrom over time; a flexible reservoircommunicating with the test chamber and carrying a storage liquid; and aspring-bias device for compressing the flexible reservoir, to pressurizethe test chamber with the storage liquid, such that additional storageliquid is supplied to the test chamber as liquid escapes therefrom overtime.
 8. Sensor apparatus as defined in claim 7, and further comprisinga one-way check valve located between the flexible reservoir and thetest chamber, for limiting the flow of liquid from the test chamber tothe flexible reservoir.
 9. Sensor apparatus as defined in claim 7,wherein:the test chamber and the flexible reservoir define a closed,sterile system; and the sensor apparatus further comprises an outercontainer for carrying the housing, the flexible reservoir, and thespring-bias device, the outer container being substantially liquidimpermeable.
 10. Sensor apparatus as defined in claim 7, wherein thespring-bias device comprises:a substantially rigid body defining acavity sized to carry the flexible reservoir; and a spring-biasedplunger for compressively engaging the flexible reservoir.
 11. Sensorapparatus as defined in claim 7, wherein the spring-bias devicecomprises a spring clip sized to compressively grasp opposite sides ofthe flexible reservoir.
 12. Sensor apparatus as defined in claim 7,wherein the components of the sensor apparatus are sufficientlypermeable to allow limited escape of the storage liquid therefrom overtime and to allow air to be introduced into the test chamber over time.13. A blood chemistry sensor assembly comprising:a housing defining atest chamber; a plurality of sensors carried within the housing, inoperative relationship to the test chamber, for measuring theconcentration of predetermined parameters of blood introduced into thetest chamber; wherein the sensor assembly is stored with a storageliquid in its test chamber; and wherein components of the sensorassembly are sufficiently permeable to allow limited escape of thestorage liquid from the test chamber over time and to allow air to beintroduced into the test chamber over time; a flexible bag that carriesa supply of storage liquid, in liquid communication with the testchamber; and a spring-bias device for pressurizing the flexible bag,such that additional storage liquid is supplied to the test chamber asliquid escapes therefrom over time.
 14. A blood chemistry sensorassembly as defined in claim 13, wherein the spring-bias devicecomprises:a substantially rigid body defining a cavity sized to carrythe flexible bag; and a spring-biased plunger for compressively engagingthe flexible bag.
 15. A blood chemistry sensor assembly as defined inclaim 13, wherein the spring-bias device comprises a spring clip sizedto compressively grasp opposite sides of the flexible bag.